Compressors, such as those used in turbochargers, typically include a compressor wheel that is rotatably mounted in a housing and that defines blades extending radially outward in proximity to an inner surface of the housing. The housing defines an inlet for receiving air or other gas in an axial direction, and an annular diffuser extends circumferentially around the wheel to receive the air in a radial direction therefrom. A volute, disposed radially outward from the diffuser, is structured to receive the air from the diffuser. During operation, the air is compressed by the rotation of the blades of the compressor wheel and delivered radially outward through the diffuser to the volute. The compressor wheel is normally rotated at a high speed, such that the air is moved at a high velocity to the diffuser, which then slows the air and increases the static pressure of the air. In the volute, which provides a relatively large volume compared to the diffuser, the velocity of the air is further reduced and the static pressure of the air is also increased.
In the case of a compressor with multiple sequential stages, the air can be compressed to a first pressure in a first stage and then further compressed to a higher pressure in a second stage. For example, U.S. Pat. No. 6,062,028 to Arnold, et al., issued May 16, 2000 and assigned to the assignee of the present application, describes a “Low Speed High Pressure Ratio Turbocharger,” which can have a two-stage compressor with back-to-back radial flow compressor wheels with integral air flow ducting. The rotational speed of the turbocharger can be reduced and/or the pressure ratio can be increased relative to single-stage devices.
While conventional two-stage compressors have been proven to be effective and to provide advantages over some single-stage compressors, there exists a continued need for an improved compressor that can be used to sequentially compress gas in multiple stages. In particular, the compressor should provide a smooth flow of gas from the outlet of the first stage to the inlet of the second stage, preferably while reducing the swirling of the gas exiting the first stage and/or while reducing the transfer of thermal energy between the gas exiting the second stage and the air flowing from the first stage to the second stage.